Existing connectors for optical fibers suffer from several limitations that reduce their effectiveness for precision capture and transfer of light in optical delivery systems, particularly high power laser systems. Fundamental to any such system is the ability to precisely position the fiber at the focus of a laser beam in free space (X, Y and Z planes.).
Prior Art FIG. 1 defines the initial application of focusing light into a fiber. A focusing objective lens 100 focuses the light from a laser to a spot 102 closely matching the diameter of the core 104 of fiber 106. This can be as small as 2 to 3 microns in diameter. The fiber then has to be positioned so the end face of the core is at the precise location of the focused spot.
Conventional methods use bulky XYZ translation stages to position the fiber and/or the lens in free space to align the fiber core with respect to the focused spot. Such stages are expensive, and are not applicable to systems where the fiber must be aligned, and semi-permanently attached.
Other methods have been employed where either the fiber is permanently attached to a lens or it is positioned at a fixed distance Z relative to the lens. The fiber/lens assembly is then manipulated and fixed relative to the laser beam in the remaining 5 planes (X, Y, pitch, yaw and roll). U.S. Pat. Nos. 4,753,510 and 4,889,406 of Omur M. Sezerman disclose a tilt-adjustable connector that can be used for such manipulation. The positioning of the fiber in the Z-plane is normally done by terminating the fiber in a conventional fiber connector (see Prior Art FIG. 2). The connector 108 is plugged into a receptacle 110 where it makes contact with a stop 112. The connector ferrule 114 and sleeve 116 are manufactured to a high degree of precision, restricting the fiber in the X-Y plane. The lens 118 is precisely positioned with respect to the receptacle 110 so that the tip of the fiber is positioned at the focal plane of the lens. Assuming that the laser beam entering the lens is perfectly collimated (ie: the laser beam waist location is well within the Rayleigh distance ZR from the focusing lens), then the focused spot will be at the same distance Z from the lens as the fiber, and only adjustments in the remaining 5 planes are necessary.
Note the existence of a key 120 on the connector 108 and keyway 122 on the receptacle 110. This feature allows one to maintain the angular orientation of the fiber (i.e: to control roll). This is necessary for certain applications, such as working with polarization maintaining fibers or with fibers with angled end faces.
The limitation of this technique is that if the laser beam is not well collimated the focused spot will not lie at the focal plane of the lens, and thus it will not lie at the tip of the fiber. Therefore for improved alignment, one needs a way to precisely adjust the distance between the fiber and lens during alignment, preferably without affecting the location of the fiber in the other five axes (X, Y, pitch, yaw, and roll).
One approach to adjust the distance is to move the lens. This suffers from two drawbacks. First the lens is between the fiber and the laser, and is often thus inaccessible. The other is that moving the lens along the Z axis usually causes unwanted motion (play) in the other planes, particularly X and Y.
Another idea is to simply mount the fiber in a threaded tube, and screw the fiber into a mating threaded receptacle. This has the drawback of being unable to control the roll of the fiber, making it unusable for polarization maintaining fiber applications.
Another issue that one wants to avoid is accidentally extending a fiber too close to a lens or other surface, possibly jamming and damaging the tip of the fiber.
An alternative to the previously described connecting systems involves the use of a compression spring within the connection device mounting the optical fiber. U.S. Pat. Nos. RE38,205E (being a reissue of U.S. Pat. No. 5,734,778) and 6,250,818 teach connectors that incorporate at least one compression spring that aids in achieving a degree of adjustment of the fiber relative to the receptacle in which it is to be received. However, in these patents the spring action is not such as to permit any compressive movement after the connector Z-position is located. This leaves open the possibility of jamming and damage to the ferrule tip should it be mated to conventional connectors or receptacles, which rely on some compressive spring action being present when mating.